In order to determine molecular weight or size distribution of separated particles in GPC (Gel Permeation Chromatography) and HDC (Hydrodynamic Chromatography), peak composition is inferred from its elution volume. Elution volume has been determined in chromatography by measuring the transit time of unretarded marker species to which the detector is sensitive and ratioing solute position to marker position. Use of a marker is quite typical since even premium quality liquid chromatographic pumps are generally not capable of better than 0.3% flow stability over repeated analyses. For this reason, the practice of assuming constant flow and measuring elution time would frequently result in unacceptable uncertainties in the determination of latex particle diameters as an illustrative example.
Another flow related source of error for concentration-sensitive detectors (UV, IR, RI, Conductivity) in LC is the inverse proportionality between peak area and flow rate, e.g., a 0.5% flow decrease produces a 0.5% area increase.
The most troublesome flow fluctuations are those with periods on the order of peak widths since these cause individual peak areas to change. Such fluctuations can occur, e.g., with reciprocating piston pumps because check valve leakage rates tend to change for subsequent pump strokes, and stroke volumes are typically 50 to 500 .mu.l.
Present methods for measuring elapsed flow include collecting a volume of eluent in a graduated cylinder, measuring the movement of a bubble injected into the flowing liquid, or accumulating the total number of dumps of a siphon dump counter, all techniques which can be somewhat imprecise or erratic.
Other classical flow measuring devices, generally for higher ranges, include the following:
1. Coriolis flow meter, measures mass flow as a function of gyroscopic torque forces. This method is complex and expensive; accuracy is .+-.0.4%. PA1 2. Ultrasonic flow meter, suited for gallons-per-minute flow; accuracy is .+-.0.5%. PA1 3. D/P flow cell, measures pressure drop across an orifice. Prone to plugging, drift; viscosity dependent. PA1 4. Turbine meter, target meter, venturi meter, rotameter, Pitot tube, all principally applicable to flow rates in excess of 50 cc/min. PA1 5. Continuous heat addition flow meter, heats eluent and measures downstream temperature continuously. Result varies with the specific heat of the metered liquid and ambient temperature fluctuations. PA1 6. Self-heating thermistor, undergoes cooling proportional to flow. Nonlinear and result varies with specific heat of solution and ambient temperature variations. PA1 1. minimizing the thermal mass of the heat "pulser" and sensor through the application of semiconductor pulsing and sensing elements; PA1 2. electronically time-differentiating the sensor output to reject characteristically slower ambient thermal drift and to minimize response time in preparation for subsequent pulse detection; PA1 3. application of a flow metering scheme which uses an improved method for high precision flow measurements and flow cell calibration; and PA1 4. development of a flow cell and method, which by component selection and operation, is highly independent of temperature and liquid composition variables.